It is known that synchronisation is an essential procedure in a modern digital communication system. It is the procedure used by a remote unit (often referred to as User Equipment, UE, in UMTS or Customer Premises Equipment, CPE) to identify valid transmissions from infrastructure equipment (often referred to as Node Bs in UMTS) and align the remote frequency reference and timing to that used by the infrastructure.
UMTS Terrestrial Radio Access (UTRA) Time Division Duplex (TDD) and Frequency Division Duplex (FDD) modes both provide a synchronisation channel (SCH) that is used by the UE to search for valid signals and perform the synchronisation procedure. The SCH transmission consists of one real valued Primary Synchronisation Code (PSC) and three complex Secondary Synchronisation Codes (SSC), all of length 256 chips The PSC is common for all Node Bs, but the SSCs are Node B specific. The PSC and SSC are transmitted simultaneously from a given Node B at a specific fixed time offset (toffset) from the start of time slot 0. The time offset is included to prevent the possible capture effect that would otherwise occur as a consequence of all Node Bs transmitting the common primary code at the same time.
The UE uses the PSC to search for and identify transmissions from Node Bs. The PSC is also used as a reference from which the UE is able to generate a correction that can be used to correct the frequency of the UE's reference oscillator. The SSC is included to signal the additional information required by the UE in order to achieve the full time-aligned synchronization and also to begin to demodulate system information broadcast on the Broadcast Channel (BCH) which is carried by the Primary Common Control Physical Channel P-CCPCH.
For single chip-rate systems where the chip rate used by the Node B and the UE is predetermined by the system design, the synchronization procedure briefly outlined above is sufficiently complete.
However, considering a network where multi-chip rates are supported, in an initial start-up condition, the UE will not be aware of the chip rate that is available; therefore, the receiver in the UE is unable to select the correct chip-rate.
In some known systems such as those using fixed line modems, the available bandwidth is negotiated in the initial data transfers between sender and receiver.
This is done at a predetermined fixed rate, usually determined by the system design or backwards compatibility with early implementations.
Other possible schemes might transmit the whole timeslot in which SCH bursts are transmitted at the lower chip-rate (note that for a UMTS TDD system, the SCH is transmitted in every radio frame).
A plurality of subscriber terminals (or user equipment (UE) in UMTS nomenclature) 112, 114, 116 communicate over radio links 118, 119, 120 with a plurality of base transceiver stations, referred to under UMTS terminology as Node-Bs, 122, 124, 126, 128, 130, 132. The system comprises many other UEs and Node Bs, which for clarity purposes are not shown.
The wireless communication system, sometimes referred to as a Network Operator's Network Domain, is connected to an external network 134, for example the Internet. The Network Operator's Network Domain includes:
(i) A core network, namely at least one Gateway GPRS Support Node (GGSN) 144 and or at least one Serving GPRS Support Nodes (SGSN); and
(ii) An access network, namely:                (ai) a GPRS (or UMTS) Radio network controller (RNC) 136-140; or        (aii) Base Site Controller (BSC) in a GSM system and/or        (bi) a GPRS (or UMTS) Node B 122-132; or        (bii) a Base Transceiver Station (BTS) in a GSM system.        
The GGSN/SGSN 144 is responsible for GPRS (or UMTS) interfacing with a Public Switched Data Network (PSDN) such as the Internet 134 or a Public Switched Telephone Network (PSTN) 134. A SGSN 144 performs a routing and tunnelling function for traffic within say, a GPRS core network, whilst a GGSN 144 links to external
However, the above known fixed initial rate negotiation scheme and the other possible schemes have the disadvantage that they are inefficient.
A need therefore exists for a synchronisation scheme for multi-rate communication wherein the abovementioned disadvantage may be alleviated.